The present invention relates generally to an electronic closed loop air-fuel ratio control system for use with an internal combustion engine, and particularly to an improvement in such a system for optimally controlling an air-fuel mixture fed to the engine by limiting the magnitude of a reference signal within a predetermined range, the reference signal being compared with an output voltage of an exhaust gas sensor in a differential signal generator.
Various systems have been proposed to supply an optimal air-fuel mixture to an internal combustion engine in accordance with the mode of engine operation, one of which is to utilize the concept of an electronic closed loop control system based on a sensed concentration of a component in exhaust gases of the engine.
According to the conventional system, an exhaust gas sensor, such as an oxygen analyzer, is deposited in an exhaust pipe for sensing a component of exhaust gases from an internal combustion engine, generating an electrical signal representative of the sensed component. A differential signal generator is connected to the sensor for generating an electrical signal representative of a differential between the signal from the sensor and a reference signal. The reference signal is previously determined in due consideration of, for example, an optimum ratio of an air-fuel mixture to the engine for maximizing the efficiency of both the engine and an exhaust gas refining means. A so-called proportional-integral (p-i) controller is connected to the differential signal generator, receiving the signal therefrom. A pulse generator is connected to the p-i controller, receiving a signal therefrom and generating, based on the received signal, a train of pulses which is fed to an air-fuel ratio regulating means, such as electromagnetic valves, for supplying an air-fuel mixture with an optimum air-fuel ratio to the engine.
In the previously described control system, a problem has been encountered that the exhaust gas sensor generates a signal whose magnitude changes undesirably with change of atmospheric temperature, and with decrease of its efficiency due to a lapse of time. This change of the magnitude makes difficult a precise control of the air-fuel mixture ratio. In order to remove this defect, in accordance with the prior art, the magnitude of the reference signal has been changed depending upon change of a means value of the magnitude of the signal from the exhaust gas sensor.
However, in spite of this improvement, another problem has been encountered. That is, when for example, the output of the exhaust gas sensor decreases or increases due to certain causes to a considerable extent, the magnitude of the reference signal, resultantly, decreases or increases considerably. Therefore, the air-fuel mixture ratio cannot be precisely controlled for a certain period of time in that a transient time of a circuit determining the mean value cannot be neglected.